A polyarylene sulfide [PAS] resin as represented by polyphenylene sulfide is an engineering plastic excellent in heat resistance, chemical resistance, non-flammability, electrical properties and mechanical properties, and is widely used as materials for fiber, films, injection molding and extrusion. The production of a PAS resin through a process of reacting an aromatic dihalide compound and an alkali metal sulfide in an organic polar solvent, such as N-methylpyrrolidone (NMP), is described, e.g., in Japanese Patent Publication (JP-B) 52-12240. According to the process, a polymerization system in a high-temperature state is withdrawn by flushing into a vessel under a normal or reduced pressure, and is then subjected to recovery of the solvent, washing and drying to recover the resin, thereby finally obtaining a powdery resin product having particle sizes of from several μm to 50 μm. The powdery resin in a dry state is liable to be scattered as dust and is difficult to handle because of a small bulk density. Particularly, the powdery resin is liable to show inferior production efficiency because of a lowering in throughput at the time of melt-extrusion. Further, a PAS resin is generally provided with a level of melt-viscosity necessary for processing through so-called hot cross-linking in a step after the drying, so that the PAS resin is liable to exhibit inferior mechanical or physical properties than linear polymers.
Japanese Laid-Open Patent Application (JP-A)59-49232 has disclosed a process for processing particulate PPS, and in its Examples, the polymerizate is discharged into water, and precipitated polymer beads are recovered and subjected to a repetition of washing with warm water.
For a similar resin, JP-A 61-255933 has disclosed a process for treating a polymerizate slurry obtained in the polymerization step and containing a particulate PAS. The treating process includes: (1) a step wherein a polymerizate slurry containing polyarylene sulfide particles, by-produced crystalline and dissolved alkali chloride and arylene sulfide oligomers together with the liquid component principally comprising N-methylpyrrolidone, is subjected to sieving for separation into polyarylene sulfide particles and a slurry containing crystallized alkali chloride, (2) a step wherein the slurry containing crystallized alkali chloride is subjected to solid-liquid separation to recover crystallized alkali chloride, while the liquid component is distilled to recover N-methylpyrrolidone, (3) a step of washing the polyarylene sulfide particles with an organic solvent, such as acetone, and water, and (4) a step of distilling the organic solvent washing liquid to recover the organic solvent.
JP-A4-139215 has disclosed a process for washing polyarylene sulfide, wherein polyarylene sulfide particles recovered by sieving from a polymerizate slurry are successively washed with an organic solvent, such as acetone, having a lower boiling point than water miscible with water and polar organic solvent in the polymerizate slurry, and then with water.
The above-mentioned conventional processes for recovery by washing of product polyarylene sulfide particles from a polymerizate slurry involve an essential problem accompanying a principal step comprising separation and recovery of polyarylene sulfide particles from the polymerizate slurry relying on sieving or filtration.
More specifically, in any of the above-mentioned processes, an objective polymerizate slurry containing polyarylene sulfide particles obtained by reaction of an alkali sulfide source and an aromatic dihalide compound in a polar organic solvent, contains not only polyarylene sulfide particles having an average particle size on the order of 200–2000 μm, as a principal object of recovery, but also a polar organic solvent, by-produced alkali metal salt fine particles, dissolved alkali metal salt and arylene sulfide oligomer. Such a large number of components in the polymerizate slurry provide an essential difficulty in separation and recovery of product PAS particles, and particularly many problems are encountered in the above-mentioned separation-recovery process including the sieving or filtration as an essential step.
For example, the above-mentioned processes disclosed by JP-A 61-255933 and JP-A4-139215 both include a step of sieving the polymerizate slurry by using a sieve having a mesh-opening of ca.105 μm so as to recover PAS particles on the sieve while transferring the by-produced alkali metal salt fine particles to the filtrate liquid. It is inevitable to lose PAS particles having particle sizes passing through the sieve mesh-opening, and if the mesh-opening is decreased so as to reduce the loss, the separation between the PAS particles and the alkali metal salt fine particles becomes difficult and requires an excessively long time for the filtration, thus making the process commercially unfeasible. Further, the processes are also accompanied with much difficulty in recovery of the polar organic solvent that is a costly reaction solvent from the filtrate liquid containing the by-produced alkali metal salt fine particles.